The present invention relates to a ground anchorage testing system. In particular, the present invention relates to a system for applying a load impulse to ground anchorages and to a system for processing the vibrational response of the anchorages after applying such a load impulse.
Known ground anchorages are used to support engineering structures such as tunnels, mines, retaining walls, dry docks and dams. There are two major types of anchorage tendon, solid metal rod type or metal multi wire strand type. To fix an anchorage tendon in the ground, a bore hole is initially made into the sub-surface of the ground. Then, the anchorage tendon is inserted into the bore hole and a length of the part remote from the ground surface is bonded to the ground by resin, cement grout or the like. A further length of the anchorage tendon can also be subsequently bonded to the ground. A tensioning assembly is then positioned against the ground and located on the anchorage tendon so that the anchorage can be post tensioned to an appropriate tension. It will be appreciated that the tensioning assembly varies according to whether the anchorage tendon is of the rod type of multi strand type.
Over time, the post tension on the anchorage may vary for a number of reasons, for example due to gradual movement of the ground, due to sudden movement of the ground consequent to earthquakes, due to deterioration of the anchorage, due to loss of bonding etc. This variation in post tension, generally a loss of post tension, can lead to a local reduction in the support of the structure in question which can eventually build up to a failure to adequately support the structure if sufficient anchorages are affected. Thus, there is a need to assess the continuing integrity of ground anchorages.
WO-A-95/27831, in addition to describing a variety of fixing methods for ground anchorages, also describes a method of assessing the integrity of ground anchorages. An impulse plate is secured to the surf a end of a rod type anchorage and an impulse imparting apparatus is operatively associated with the impulse plate for applying a load impulse to the anchorage. The apparatus involves manually forcing a mass against the tension of a spring, locking the mass in position, and then manually releasing the mass so that the energy of the spring moves the mass to impact the impulse plate thereby causing the anchorage tendon to vibrate. The resultant vibrational response is compared with a reference response from the site of the anchorage at an earlier stage of its life to determine changes in the condition of the anchorage.
However, it will be appreciated that the testing of ground anchorages does not take place at a convenient or easily accessible location. Accordingly, the manual operation of the impulse imparting apparatus is difficult, cumbersome and time consuming. Moreover, the known impulse imparting apparatus is not applicable to multi strand type anchorage tendons. In fact, there is a need to use longer ground anchorages, particularly with respect to withstanding seismic loading, but it has been found that to test ground anchorages in excess of for example 10 metres, the known apparatus can not sufficiently vibrate the anchorage tendon to give a useful vibrational response. Thus, there is a need to apply larger load impulses. Nevertheless, whilst the load impulse applied must be adequate to vibrate the anchorage tendon sufficiently, care must be taken that the load impulse is not then so great as to potentially induce damage to the anchorage.
Accordingly, there is a need for an impulse imparting apparatus which is simple to use, which enables easy variation in the size of the load impulse applied so that the apparatus is versatile in application, and which can be used to apply an impulse load to a multi strand anchorage.
The aforementioned method of assessing the integrity of ground anchorages provides a simple comparison of a vibrational response with a reference vibrational response in order to assess the ground anchorage. In fact, it would be useful to be able to have a more discriminatory assessment of the ground anchorage in terms of the degree of change in post tension. In particular, it would be useful to have an accurate assessment of that change, within say 10% accuracy. The method of assessing the integrity of ground anchorages in WO-A-95/27831 does not provide this.
In addition, as anchorages get longer, it has been found that the accuracy and hence effectiveness of the techniques applied to shorter anchorages is significantly reduced. In particular, for longer ground anchorages, it is increasingly difficult to differentiate between the vibrational responses of the anchorage tendons. Thus, there is a need to improve the differentiation of the vibrational responses for longer ground anchorage tendons.
It is an object of the present invention to provide a ground anchorage testing apparatus having an impulse imparting apparatus which can impart a load impulse appropriate to longer ground anchorages, which can be easily used, and which can easily vary the magnitude of the imparted load impulse.
It is also an object of the present invention to provide a method of assessing the integrity of ground anchorages in which the resolution of the assessment of the vibrational response of the anchorage tendon can be improved sufficiently to provide an improved resolution, particularly in respect of longer ground anchorage tendons.
According to a first aspect of the present invention there is provided a ground anchorage testing arrangement having an impulse imparting apparatus connectable to a ground anchorage tendon or element thereof to be tested, the impulse imparting apparatus comprising:
an attachment means for attachment to the ground anchorage tendon;
a movable mass;
a guide for guiding movement of the mass in a direction substantially aligned with the axis of the ground anchorage to be tested;
drive means for imparting a driving force to move the mass in said direction.
Preferably, the impulse imparting apparatus comprises a control means for actuating the start of said driving force.
In a preferred embodiment, said driving force is variable.
Conveniently, the driving force is provided by a fluid.
In one embodiment, the fluid comprises compressed air.
It is preferred that the guide comprises an inner cylinder for encircling the anchorage tendon to be tested and a coaxial outer cylinder, and that the movable mass comprises an annular mass with an outer diameter substantially the same as the inner diameter of the outer cylinder and with a central aperture diameter substantially the same as the outer diameter of the inner cylinder.
According to a second aspect of the present invention there is provided a method of assessing the integrity of ground anchorages, the method comprising the steps of:
(a) imparting a load impulse to a ground anchorage tendon to be tested;
(b) monitoring the vibrational response signal of the anchorage to the imparted load impulse;
(c)conditioning the vibrational response signal;
(d) applying the conditioned vibrational response signal to an artificial neural network.
Preferably, step (c) comprises statistical analysis using one or more of the techniques of principal component analysis, wavelet transforms, and higher order spectral analysis.
It is preferred that the method further comprises the step (e) of storing the output from the artificial neural network characteristic of the Conditioned response signal.
Conveniently, the steps (a) to (e) are repeated for a plurality of different tensions on the ground anchorage to provide calibrated characteristics of the output stored in step (e).
In another preferred embodiment, the artificial neural network compares an input conditioned vibrational response signal with calibrated characteristics.